Airflow resistance is a measurement of a characteristic of an animal which impedes airflow into and out of lungs. Generally, airflow resistance is the quotient of pressure loss and airflow.
A known device for determining airway resistance is shown in U.S. Pat. No. 4,220,161 (Berlin) In this device, a rotating screen includes sections having greater and lesser resistance, and as these sections rotate across a tubular air passage, the pressure and flow rate are perturbed. Airway resistance is calculated by relationships set forth in the patent which are dependent upon variables which can be measured.
The airway perturbation device described in U.S. Pat. No. 4,220,161 and other known resistance measuring devices such as the forced-oscillation technique described in that patent are small-signal devices. That is, the resistance determined by the techniques which generally employ these instruments is an instaneous resistance ordinarily represented by a line tangent to the pressure-flow curve.
The pressure-flow curve is nonlinear, and this presents a problem when using a small-signal device to produce large-signal measurements. For example, a body plethysmograph produces measurements of resistance over pressure and flow variations much larger than that utilized in an airflow perturbation device, and this results in markedly different results. Because the body plethysmograph is a known, standard apparatus, it is desirable to convert measurements made by a small-signal device such as the airflow perturbation device shown in U.S. Pat. No. 4,220,161 to large-signal measurements.